The present invention relates to a control valve of a variable displacement compressor used for vehicle air conditioning systems.
Generally, variable displacement compressors include a displacement control valve, which is located in a control passage for connecting the discharge chamber to the crank chamber. The displacement control valve controls the opening size of the control passage. This changes the amount of high-pressure refrigerant gas supplied from the discharge chamber to the crank chamber and adjusts the pressure in the crank chamber. The difference between the pressure in the crank chamber and the pressure in the cylinder bore changes when the pressure in the crank chamber changes. The inclination angle of a swash plate is varied in accordance with this pressure difference, which varies the displacement amount.
Japanese Unexamined Patent Publication No. 9-268973 describes a displacement control valve used for a variable displacement compressor. As shown in FIG. 7, a valve chamber 101 is formed substantially in the center of the control valve. The valve chamber 101 is connected to a crank chamber 121 through a valve hole 102 and the downstream side of a control passage 120. The valve chamber 101 is also connected to the discharge chamber 123 through the upstream side of the control passage 120. A valve body 103 is accommodated in the valve chamber 101 and opens and closes the valve hole 102. An opener spring 104 urges the valve body 103 to open the valve hole 102.
A pressure sensitive chamber 105 is connected to a suction pressure zone 125. A bellows 106 is accommodated in the pressure sensitive chamber 105. A bellows spring 107 is arranged in the bellows 106. The bellows spring 107 sets the initial length of the bellows 106. The distal end of a pressure sensitive rod 108, which extends from the valve body 103, engages the bellows 106. The pressure sensitive chamber 105 is exposed to the suction pressure Ps. Therefore, when the suction pressure Ps increases, the bellows contracts. Then, the valve body 103 is moved axially to close the valve hole 102 through the pressure sensitive rod 108. When the suction pressure Ps in the suction zone 125 decreases, the bellows expands, and the valve body 103 is moved to open the valve hole 102 through the pressure sensitive rod 108.
A fixed iron core 110 is axially adjacent to the valve chamber 101. A plunger chamber 109 is axially adjacent to the iron core 110, such that the iron core 110 separates the plunger chamber 109 from the valve chamber 101. A movable iron core 111 is accommodated in the plunger chamber 109. A follower spring 112 is located between the movable core 111 and the inner bottom of the plunger chamber 109. A rod guide hole 113 is formed between the plunger chamber 109 and the valve chamber 101 to pass through the fixed core 110. A solenoid rod 114, which extends from the valve body 103, is inserted in the guide hole 113. The distal end of the solenoid rod 114 is urged against the movable core 111 by the force of the opener spring 104 and the follower spring 112. Accordingly, the movable core 111 and the valve body 103 move together through the solenoid rod 114.
Electromagnetic attraction force between the cores 110, 111 varies in accordance with the electric current supplied to a coil 115. The movable core 111 urges the valve body 103 toward the closing position with a force determined by the electromagnetic attraction through the solenoid rod 114. Accordingly, the amount of suction pressure required to close the valve changes in accordance with the level of current delivered to the coil 115.
The pressure of a discharge chamber 123 (discharge pressure Pd) is applied to the valve chamber 101 through a control passage 120. Accordingly, the valve body 103 is exposed to the relatively high discharge pressure Pd. Also, the cross sectional area of the solenoid rod 114 is equal to that of the valve hole 102. Therefore, the force of the discharge pressure Pd that urges the valve body 103 to the closing position is equal to the force of the discharge pressure Pd that urges the valve body 103 to its opening position. Therefore, the influence of the discharge pressure Pd to the valve body 103 is negated.
The pressure Pc in the crank chamber 121 is applied to the valve hole 102 through the control passage 120. The crank pressure Pc is further applied to the plunger chamber 109 through a small chamber 116c, a plunger passage 116b and a plunger groove 116a. Accordingly, the pressure in the plunger chamber 109 is equal to the crank pressure, which is the same as the pressure in the valve hole 102. The plunger passage 116b and the plunger groove 116a are formed in the control valve. The small chamber 116c is formed between a housing of the compressor and the control valve, which is installed in the compressor housing.
The crank pressure Pc in the valve hole 102 urges the valve body 103 toward its open position. The crank pressure Pc in the plunger chamber 109 urges the valve body 103 toward its closed position through the solenoid rod 114. Therefore, the force of the crank pressure Pc opening the valve body is substantially equal to the force of the crank pressure Pc closing the valve body 103. Thus, the crank pressure Pc has little influence on the valve body 103.
In the control valve of FIG. 7, the influence of the discharge pressure Pd and the crank pressure Pc is substantially nil. Therefore, there is no need to increase electromagnetic attraction force between the fixed core 110 and the movable core 111 to move the valve body 103 against the forces of the discharge pressure Pd and the crank pressure Pc. Furthermore, the valve body 103 sensitively reacts to the movement of the bellows 106 in accordance with the changes of the suction pressure Ps. Accordingly, even if the value of electric current supplied to the coil 115 is small or the change of the suction pressure Ps is slight, the opening size of the valve hole 102 is precisely controlled by the valve body 103.
The forces applied to the control valve of FIG. 7 will now be described with reference to FIG. 8. The forces applied on the valve body 103 are represented by the following expression (11). ##EQU1##
S.sub.1 represents the pressurized area of the bellows 106.
S.sub.2 represents the cross sectional area of the pressure sensitive rod 108.
S.sub.3 represents the cross sectional area of the valve hole 102.
S.sub.4 represents tho cross sectional area of the solenoid rod 114.
F represents the electromagnetic attraction force generated by the passage of electricity in the coil 115.
f.sub.0 represents the force of the bellows spring 107 and the force of the bellows 106.
f.sub.1 represents the force of the opener spring 104.
f.sub.2 represents the force of the follower spring 112.
Ps represents the suction pressure (pressure in the pressure sensitive chamber 105).
Pc represents the crank pressure (pressure in the valve hole 102).
Pd represents the discharge pressure (pressure in the valve chamber 101).
Px represents the pressure in the plunger chamber 109.
As shown above, the cross sectional area S.sub.4 of the solenoid rod 114 is equal to the cross sectional area S.sub.3 of the valve hole 102. Further, the pressure Px in the plunger chamber 109 is equal to the pressure in the valve hole 102, which is equal to the crank pressure Pc. Accordingly, since Pd(S.sub.3 -S.sub.4)=0 and -S.sub.3 *PC+S.sub.4 *Px=0 in the right side of the equation (11), the following expression (12) is obtained. EQU f.sub.0 -S.sub.1 *Ps-S.sub.2 (Pc-Ps)=F+f.sub.2 -f.sub.1 EQU Ps=(f.sub.0 +f.sub.1 -f.sub.2 -F-S.sub.2 *Pc)/(S.sub.1 -S.sub.2)(12)
The expression (12) indicates that the influence of the discharge pressure Pd on the valve body 103 is nil and that the influence of the crank pressure Pc on the valve body 103 is small.
However the control valve of FIG. 7 has the following problem.
The valve chamber 101 is exposed to the discharge pressure Pd, and the plunger chamber 109 is exposed to the crank pressure Pc. When gas from the valve chamber 101 leaks to the plunger chamber 109 through the space between the solenoid rod 114 and the rod guide 113, the pressure Px in the plunger chamber 109 becomes higher than the crank pressure Pc. Then, in the expression (11), -S.sub.3 *Pc+S.sub.4 *Px is greater than zero. Therefore, the force required to move the valve body 103 to its closed position increases. Therefore, the movement of the valve body 103 to the opening position is hindered, which prevents optimal operation of the valve body 103 in accordance with the changes of electric current supplied to the coil 115 and the changes of the suction pressure Ps.
Accordingly, it is necessary to seal the space between the solenoid rod 114 and the rod guide hole 113 to prevent leakage of gas from the valve chamber 101 to the plunger chamber 109. This requires that the solenoid rod 114 and the guide hole 113 be manufactured with high precision, thus increasing the manufacturing costs.
Also, the passage 116 must be formed in the control valve to introduce the crank pressure Pc to the plunger chamber 109. This also increases the manufacturing costs.